The present invention relates to a casting alloy of cobalt, chromium and molybdenum and containing up to 2% silicon, up to 5% manganese, and up to 1% carbon, to be used for surgical and dental prostheses.
It is state of the art to use stainless steels, Co-Cr-Mo alloys, Co-Cr-W-Ni alloys, unalloyed titanium or Ti-Al-V alloys as the metallic material for surgical implants. For dental prostheses, the above mentioned Co-Cr-Mo alloys as well as Co-Cr-Ni-Mo alloys and Co-Ti-Cr alloys are the most commonly used materials. In addition, noble metals and their alloys can be used for dental prostheses. For use in the human body, all metallic materials must meet high requirements. They should not only have good mechanical properties and particularly good corrosion resistance, but should also be of such composition that under the conditions prevailing in the human body they will not cause toxic tissue reactions. Furthermore, such metallic materials must be easily worked into complicated shapes that conform to the human body and should not be too expensive.
The known metallic materials do not always meet these requirements to a sufficient degree. For example, breaks have occasionally occurred in surgical implants and dental prostheses made of such materials as a result of insufficient ductility and permanent stresses. The onset of corrosion and toxic tissue reactions have also been noted in their use. The use of noble metals is correspondingly costly.
In order to improve the mechanical properties of implant casting alloys on a cobalt-chromium-molybdenum base, it has already been proposed to increase the nitrogen content beyond the amount of nitrogen that is usually present as an impurity, but to limit the total amount of carbon plus nitrogen to 0.7%, as disclosed in German Auslegeschrift No. 2,225,577, corresponding to U.S. Pat. No. 3,865,585. It is also known, as disclosed in Technische Rundschau Sulzer, 1974, pages 235 to 245, that the properties of Fe-Cr-Ni-Mo alloys can be influenced by a suitable heat treatment. Such heat treatment however, has the drawback when used with the conventional alloys of this type, that the improvement in ductility (elongation at rupture) as compared to non-heat treated Co-Cr-Mo alloys disclosed in the Technische Rundschau Sulzer article is connected with a worsening of the strength values, such as the values for 0.2 elastic limit, tensile strength, and hardness as compared to non-heat treated Co-Cr-Mo alloys disclosed in the Technische Rundschau Sulzer article. (Compare Tables 4 and 5 at page 237 of the Technische Rundschau Sulzer article). Similar results were to be expected with nitrogen containing Co-Cr-Mo alloys subjected to heat treatment.